Early Smelting and Metallurgy
Early Smelting and Metallurgy
As we have seen, the invention of metallurgy involved at first the simple beating of malleable copper into the shape desired. It was discovered that heating, called annealing, made copper more malleable, a process called tempering. The copper was hammered after it was heated. The resulting edge on a tool or weapon was harder and more durable.
Shaping of copper was also accomplished by cutting or grinding with abrasive stones, with such hard minerals as corundum.
Most copper is not found in its native state, that is, as a pure reddish brown metal. Usually, the copper is combined with other substances to make a mineral. Minerals may include the purplish cuprite, a combination of copper and oxygen, but most often, it is a green or blue mineral where copper is combined with other substances besides oxygen. When copper is in a mineral, it cannot be used as copper. It cannot be hammered or bent into another shape. Smelting copper ore to separate the copper from copper minerals required a hotter fire than normal cooking fires. It also required easily melted copper-bearing ore minerals such as green malachite and blue azurite or purplish red cuprite. The hotter fire was known by some cultures already, due to the development of fired pottery in a kiln. Probably by accident, it was discovered that the blue or green or purplish stones could be reduced to liquid copper metal in a fire. The fire must be hotter than a normal campfire, so there are two likely explanations for the discovery of smelting:
 Azurite |
 Malachite |
 Native Copper |
 Cuprite |
The primitive process of extracting metallic copper from copper-bearing stones by melting it is called smelting. Working with metal to extract it from ore and to form it into useful objects is called Metallurgy.
Copper was extracted from ore such as the mineral malachite. Ore is a mineral or aggregate of minerals from which a substance, usually a metal, can be extracted at a profit.
Shaping copper by casting it, that is, by pouring molten liquid copper into a mold, also first happened before anyone wrote it down, or even could write it down. Perhaps a bit of liquid metal ran into a footprint in the dirt by a campfire, where it formed a perfect cast of the footprint.
Smelting was used as well to make bronze, an alloy. Alloys are mixtures of metals. Bronze is an alloy of copper and tin.
Smelting was accomplished initially in an open fire. A hole
one to two feet in diameter was dug in the ground. The hole was
lined with fire-resistant clay or stone. 
Charcoal was
placed in a layer on top of the clay, then was covered by copper
ore. Charcoal burns particularly hot, raising temperatures enough
to melt the copper. The charcoal also releases gases that react
with the copper minerals to reduce them to copper metal. A molten
mass of the dense copper formed, topped by lighter waste
products, or slag. When the mass was cool, the
brittle, glassy slag could be broken off, leaving a cake of
refined copper behind. The furnace in this method is called an open
hearth. Remnants of open hearths have been found in
Sumerian ruins.
A more complex method used the crucible, a ceramic pot into which the charcoal and copper ore were placed. The crucible was placed into a fireplace, or furnace. Bellows, often made of goat skins, directed a draft of air to raise the temperature. The copper cake produced by this method was cleaner than that in the open hearth. Copper ingots were cast into the shape of bars, rings, and other shapes suitable to be transported to Sumerian cities.
Geology
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